1. Field of the Invention
This invention relates to a system for driving a brushless motor used in an audio device, a visual display device and the like.
2. Description of the Prior Art
High reliability and a long useful service life are required for a motor used in an audio device, a visual display device and the like. In order to satisfy the above requirements, it is a recent tendency to employ, in place of a conventional d.c. motor having brushes and a commutator, a so-called brushless motor in which a position sensor is provided to detect the angular position of rotation of a rotor and, on the basis of the output signal of the position sensor, a plurality of drive transistors connected in series with a plurality of drive coils of a plurality of phases respectively are sequentially turned on so as to continuously rotate the rotor.
The structure and operation of a prior art brushless motor driving system will be described with reference to FIGS. 10 to 13.
FIG. 10 is a block circuit connection diagram of the prior art brushless motor driving system. Referring to FIG. 10, drive coils 401a, 401b and 40lc of a brushless motor are connected in series with drive transistors 403a, 403b and 403c respectively. The drive transistors 403a, 403b and 403c are grounded at their emitters. These drive transistors 403a, 403b and 403c are connected at their collectors to a power supply terminal 402 through the drive coils 401a, 401b and 40lc respectively. Further, the drive transistors 403a, 403b and 403c are connected at their collectors to inverted input terminals of comparators 405a, 405b and 405c respectively. The power supply terminal 402 is also connected to non-inverted input terminals of the comparators 405a, 405b and 405c. These comparators 405a, 405b and 405c are connected at their output terminals to corresponding input terminals respectively of a differentiation circuit 408, and this differentiation circuit 408 is connected at its output terminals to corresponding input terminals respectively of a delay circuit 409. The delay circuit 409 is connected at its output terminals to corresponding input terminals respectively of an energization switching circuit 410, and a forward-backward command circuit 411 is also connected at its output terminal to a corresponding input terminal of the energization switching circuit 410. The energization switching circuit 410 is connected at its output terminals to corresponding input terminals respectively of an energization switching signal amplifier circuit 406. This energization switching signal amplifier circuit 406 is connected at its output terminals to the bases of the drive transistors 403a, 403b and 403c respectively.
The operation of the prior art brushless motor driving system having the structure described above will now be described with reference to FIGS. 11 to 13. FIG. 11 shows signal waveforms appearing when the brushless motor in the system shown in FIG. 10 is rotating in the forward rotational direction, FIG. 12 shows signal waveforms appearing when the brushless motor is rotating in the backward rotational driection, and FIG. 13 shows signal waveforms appearing when a backward rotation command signal is applied while the brushless motor is rotating in the forward rotational direction.
A power supply voltage V.sub.402 is applied to the power supply terminal 402, and voltages E.sub.401a, E.sub.401b and E.sub.401c having waveforms as shown in FIG. 11 are induced in the drive coils 401a, 401b and 401c respectively. The comparators 405a, 405b and 405c compare the voltages E.sub.401a, E.sub.401b and E.sub.401c with the power supply voltage V.sub.402 and generate output signals V.sub.405a, V.sub.405b and V.sub.405c having waveforms as shown in FIG. 11, respectively. For example, the output signal V.sub.405a of the comparator 405a comparing the voltage E.sub.401a with the voltage V.sub.402 is V.sub.405a of low level when E.sub.401a .gtoreq.V.sub.402, but it is V.sub.405a of high level when E.sub.401a &lt;V.sub.402. The differentiation circuit 408 differentiates the leading edges of the output signals V.sub.405a, V.sub.405b and V.sub.405c of the respective comparators 405a, 405b and 405c and generates pulse signals Poa, Pob and Poc having waveforms as shown in FIG. 11. These pulse signals Poa, Pob and Poc are applied to the delay circuit 409 through which they are turned into pulse signals Pa, Pb and Pc delayed by a predetermined delay time T relative to the original signals Poa, Pob and Poc respectively. When the forward-backward command circuit 411 generates a forward rotation command signal V.sub.F/R, the energization switching circuit 410 generates energization switching signals Va, Vb and Vc having waveforms determined by the pulse signals Pa, Pb and Pc supplied from the delay circuit 409. That is, the energization switching signal Va is turned to high level in response to the application of the pulse signal Pa, but it is turned to low level in response to the application of the pulse signal Pb. The energization switching signal Vb is turned to high level in response to the application of the pulse signal Pb, but it is turned to low level in response to the application of the pulse signal Pc. The energization switching signal Vc is turned to high level in response to the appliecation of the pulse signal Pc, but it is turned to low level in response to the application of the pulse signal Pa.
When the motor is rotating in the forward rotational direction, the energization switching signals, Va, Vb and Vc provide optimum energization timing with respect to the induced voltage signals E.sub.401a, E.sub.401b and E.sub.401c. These energization switching signals Va, Vb and Vc are applied, after having been amplified through the energization swithcing signal amplifier circuit 406, to the bases of the respective drive transistors 403a, 403b and 403c, so that the motor continues to efficiently rotate in the forward rotational direction.
On the other hand, when the forward-backward command circuit 411 generates a backward rotation command signal V.sub.F/R, and the motor rotates in the backward rotational direction, signal waveforms as shown in FIG. 12 appear in the system. In this case, the energization switching signal Va is turned to high level in response to the application of the pulse signal Pa, but it is turned to low level in response to the application of the pulse signal Pc, the energization switching signal Vb is turned to high level in response to the application of the pulse signal Pb, but it is turned to low level in response to the application of the pulse signal Pa, and the energization switching signal Vc is turned to high level in response to the application of the pulse signal Pc, but it is turned to low level in response to the application of the pulse signal Pb.
In this case too, the energization switching signals Va, Vb and V provide optimum energization timing with respect to the induced voltage signals E.sub.401a, E.sub.401b and E.sub.401c, so that the motor continues to efficiently rotate in the backward rotational direction like in the case of the rotation in the forward rotational direction.
However, a problem such as pointed out below has been encountered in the prior art brushless motor driving system. That is, when the forward-backward command circuit 411 generates a backward rotation command signal V.sub.F/R while the motor is rotating in the forward rotational direction, the switching signals may not act to produce torque which is of the opposite direction with respect to the rotational direction of the rotor, and the rotational direction of the motor may not be changed. Also, because the delay circuit 409 employed in the prior art brushless motor driving system includes means such as a monostable multivibrator providing a fixed delay time, the system has not been applicable to a brushless motor whose rotational speed is variable over a wide range. Further, the prior art brushless motor driving system has such an additional problem that the motor cannot be started from a standstill state because voltages are not induced in the drive coils of the individual phases in such a state of the motor.